Disorders of calcium and bone metabolism

Clinical information

The activity of bone-forming osteoblasts and bone-resorbing osteoclasts, calciotropic hormones and extracellular calcium and phosphate concentrations all have a significant influence on bone metabolism. While osteoblasts provide for bone tissue formation by excreting calcium phosphate and collagen, osteoclasts are responsible for extracellular-matrix degradation and remodelling. Cell activity and thus also the calcium metabolism is controlled by hormones, growth factors and cytokines. The most important regulators are parathyroid hormone (PTH), calcitonin and vitamin D.

Balanced homoeostasis is important for the maintenance and continuous formation of bone. Diseases that impact the bone metabolism often lead to skeletal deformations, weak bone structure and muscoskeletal pain.

Diagnostics

An imbalance of this complex regulation network (e.g. hyper- or hypoparathyroidism or hypovitaminosis D) can have many causes and lead to various diseases.

Hyperparathyroidism (HPT) is characterised by an increased PTH level in the blood. Secondary complications of HPT include decreased bone density, osteoporosis and kidney stones.

In hypoparathyroidism, the PTH concentration is too low. It is often caused by thyroid surgery in which the parathyroid is also removed – sometimes unintentionally. The resulting calcium deficiency can lead to severe muscle cramps and even cardiac insufficiency.

Hypovitaminosis D means a severe vitamin D deficiency. This is most commonly caused by insufficient sun exposure of the skin and a diet low in vitamin D. The undersupply of the body with vitamin D can have severe consequences, e.g. a secondary rise of PTH concentration due to calcium deficiency, bone softening or atrophy. Determination of the total amount of circulating 25-OH vitamin D, the stored form of vitamin D, is particularly suited to evaluate the status of vitamin D supply. In patients with chronic kidney disease (CKD) and vitamin D deficiency, the levels of 25-OH vitamin D, PTH and calcium should be monitored.

The measurement of active vitamin D, i.e. the concentration of 1,25(OH)21,25(OH)2 vitamin D, supports the diagnosis of different congenital and acquired calcium metabolism disorders, as they are associated with changes in the renal and extrarenal production of the active form of vitamin D.